Solid oxide fuel cells (solid oxide fuel cell, SOFC) are used to convert the chemical energy of a combustible gas together with an oxidizer, e.g., atmospheric oxygen, on the direct way electrochemically into electric energy. See, e.g. DE 102 38 860 A1, but also to EP 1 271 684 A1 (U.S. Pat. No. 6,737,182) the disclosures of which are incorporated herein by reference.
The conversion of the combustible gas and the atmospheric oxygen into electric energy is conducted on and in ceramic layers (cathode, electrolyte, anode). In planar fuel cells a cell consists of a planar configuration of the ceramic layers. So-called bipolar plates are used to supply the combustible gas and the air, to dissipate the residual gases and to provide an electric connection between the individual fuel cells, which are stacked one above the other, in a serial arrangement, that is, in so-called stacks. The ceramic layers may also be a part of these bipolar plates. At the same time it must be guaranteed that the combustible gas and the air do not make direct contact with each other at any point in the stack of fuel cells. In particular, the bipolar plates may form, together with the ceramic layers, a cassette, which encloses one type of gas, in particular the combustible gas.
The operating temperature of solid oxide fuel cells ranges from 600 degrees C. to 900 degrees C. The temperature of solid oxide fuel cells is usually raised relatively slowly to their operating temperature to avoid damage due to the occurrence of thermomechanical stress between the ceramic layers with each other and/or between the ceramic composite and the bipolar plates. Thermomechanical stresses between the ceramic layers with each other and/or between the ceramic composite and the bipolar plates may lead, in particular, to micro-cracks in the ceramic layers as well as in the ceramic-ceramic interfaces and between the ceramic-metal interfaces and, thus, to the destruction of the SOFC.
For use in motor vehicles, fuel cells with very short start up times are necessary. One approach is to apply the ceramic functional layers not in a self bearing manner (e.g., the electrolyte or the anode as a substrate), but rather as thin layers on a metal substrate (e.g., sintered metal or a perforated foil, see, for example, DE 102 38 860 A1). In addition to the thermomechanical stability, another limiting factor is the introduction of the necessary amount of heat. EP 1-271 684 A2 describes the possibility of raising the temperature of a solid oxide fuel cell to its operating temperature of an electric resistance heating element. In this patent, metal foils, which are provided between the ceramic layers, are put under electrical stress, thus generating heat by the resulting current flow. However, this solution requires an additional component, whose integration and contacting in the stack means a significant increase in the complexity of such a SOFC. Therefore, the described method runs the risk that, when the exterior side of the metal foil or of the other metal components is not electrically insulated, electric short circuiting ensues over the bipolar plates or the electrodes of the SOFC, thus electrically bridging the “heating foil” and rendering it inactive.
It shall hereby be provided now a solid oxide fuel cell comprising a metal bearing structure, which exhibits passage orifices for a gas and is intended for a cathode-electrolyte-anode unit, and comprising a bipolar plate, which is provided on the other side of the bearing structure, or the like. With respect to raising the temperature of said cell, said cell can be electrically heated and yet is provided by a simple and reliable construction.